The present invention relates to a voltage non-linear resistor used mainly in the field of electric power and including a main component of ZnO. The invention also relates to a method of fabricating such a voltage non-linear resistor.
Non-linear resistors made of a main component of ZnO (ZnO element) have an excellent non-linear characteristics and are widely used as elements for arresters. The ZnO element is fabricated by adding a small amount of metallic oxides such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, MnCO.sub.3, Cr.sub.2 O.sub.3, Co.sub.2 O.sub.3, B2O.sub.3, Al(NO.sub.3).sub.3 to a main component of ZnO, mixing and granulating the oxides, compacting the mixture, then sintering and heat-treating the compacted body, the sintered body being provided with an electrode.
Following are definitions of terms used to describe characteristics of ZnO elements of the type contemplated by the present invention:
LIMITING VOLTAGE: A terminal voltage of a ZnO element when current n A flows through the element.
FLATNESS: A ratio of a terminal voltage (V.sub.5kA) of a ZnO element when current of 5000 A flows through the element to a terminal voltage (V.sub.1mA) when current of 1 mA flows. EQU Flatness=V.sub.5kA /V.sub.1mA
WITHSTANDING INPUT ENERGY: A total input energy (E) per unit volume of ZnO element when current of 2 ms*IA is supplied to the ZnO element repeatedly N times until causing failure. EQU E=(2.times.10.sup.-3 .times.I.times.V.times.N)/Volume of the element (cm.sup.3)
Where, V: A terminal voltage of the element when current of IA flows.
LEAK CURRENT: An effective current (AC) flows through an element when a voltage (wave height AC), which is 90% of V.sub.1mA (a terminal voltage when current of 1 mA is applied to a ZnO element at room temperature), is supplied between terminals in the element at 120.degree. C.
Very important characteristics for arresters are their discharge withstanding capacity and their voltage applying life time characteristics. Especially for ZnO elements used in a gap-less arrester, they are always in a voltage applied condition and minute leakage current occurs in the ZnO element, the leakage current gradually increasing as the voltage applied time increases. In some cases, the ZnO element is heated to cause a thermal runaway phenomenon. To prevent the ZnO element from the thermal runaway phenomenon and to thus improve its life time, it is important that the increasing rate of the leakage current decreases as the voltage applied time increases. For a ZnO element having a high limiting voltage, it is also important that the discharge withstanding capacity and the voltage applying life time characteristics are outstanding.
The limiting voltage is generally indicated by the voltage per unit thickness of ZnO element when current of 1mA flows in the ZnO element. Since the limiting voltage of a ZnO element is determined by the number of grain layers in the ZnO element existing between its electrodes, the limiting voltage depends on the grain size of the ZnO forming the sintered body when it is evaluated by unit thickness. Therefore, in order to increase the limiting voltage of a ZnO element, it is effective that the growth of grains composing the sintered body be suppressed. In the past, the method employed to suppress the grain growth has been a method having low sintering temperature or a method adding a grain growth suppressing agent such as SiO.sub.2. For example, methods in which a fairly large amount of SiO.sub.2 is added compared to a usual fabricating method are described in Japanese Patent Publication No. 55-13124 (1980) and Japanese Patent Publication No. 59-12001 (1984).
On the other hand, a method to obtain a long life element by suppressing the deterioration in characteristics due to voltage normally applying to a ZnO element is described in Japanese Patent Application Laid-Open No. 58-159303 (1983). The method to prevent the deterioration in the characteristics of the ZnO element is a so-called once-heat-treatment after sintering in which a ZnO element is sintered at a high temperature of 1050.degree. to 1300.degree. C., is heated to 500.degree. to 700.degree. C., maintained at that temperature for 1 to 2 hours, then cooled to room temperature with a cooling speed of 100.degree. to 300.degree. C./hour. Another method is described in Japanese Patent Application Laid-Open No. 58-200508 (1983) for preventing the deterioration in the characteristics of the ZnO element involving so-called twice-heat-treatment after sintering in which an element containing ZnO as a main component and at least Bi.sub.2 O.sub.3 is sintered at a high temperature of 1050.degree. to 1300.degree. C., is heated to 850.degree. to 950.degree. C. and maintained at that temperature for 1 to 2 hours, is then cooled to 300.degree. C. with a cooling speed of 300.degree. C./hour, is then re-heated to 500 to 700.degree. C., maintained at that temperature for 1 to 2 hours, and is then re-cooled to room temperature with a cooling speed of 50.degree. to 150.degree. C./hour.
It is economically effective and advantageous to increase the limiting voltage of a ZnO element since this will facilitate manufacture of an arrester for electric power distribution which can be made small in size. Accordingly, an object of the present invention is to increase the limiting voltage of a ZnO element.
One of the methods to increase the limiting voltage of ZnO elements is to suppress grain growth of ZnO by increasing the content of the additive of SiO.sub.2 to form Zn.sub.2 SiO.sub.4 during sintering. However, since the increasing rate of the limiting voltage for a ZnO element having a high content of SiO.sub.2 is small when the ZnO element is sintered through the conventional technology described above, a problem is that there is a limitation to make a substantial increase in the limiting voltage even if a great deal of SiO.sub.2 is added. Further, another problem is that adding a large amount of the SiO.sub.2 decreases the withstanding discharge capacity of the ZnO element due to local concentration of current flow since changes in the composite oxide due to reaction of SiO.sub.2 with other additives occurs to make the insulation characteristic of grain boundary precipitation non-uniform. Furthermore, in the method to suppress the grain growth of ZnO by low temperature sintering, there is a problem in that the withstanding capacity of the sintered body cannot be increased since its sintering is insufficient.
The ZnO element has a structure in which a ZnO particle is surrounded with a high resistive boundary layer and the resistance of the boundary layer has a non-linearity against voltage.
Generally, the voltage-current characteristic of a ZnO element can be expressed by the following equation. EQU I=KV.sup.60 (Equation 1)
Where I is the current, V is the voltage, K is a constant, .alpha. is a non-linear coefficient. The coefficient .alpha. for ZnO elements is approximately 10 to 70.
When the coefficient .alpha. is large, the leakage current flowing in the ZnO element under normal voltage applying condition is small. Therefore, the coefficient .alpha. is preferably large. In order to suppress the increase of leakage current due to applying voltage for a long time, it is effective that a .gamma.-type Bi.sub.2 O.sub.3 phase is formed in the ZnO element with heat-treatment of the sintered ZnO element.
However, the above-mentioned conventional technology, where a sintered ZnO element is heat-treated once at a temperature of 500.degree. to 700.degree. C., has a disadvantage in that the voltage-current characteristic of the element is inferior though the deterioration in characteristic can be suppressed by forming y-type Bi.sub.2 O.sub.3 in the ZnO element.
On the other hand, in the case to improve the life time of the ZnO elements by twice heat-treating a sintered ZnO element, there is a problem in that when the .gamma.-type Bi.sub.2 O.sub.3 is not formed in the ZnO element in the first heat-treatment, the voltage applying life time characteristic of the ZnO element does not improve even if the second heat-treatment is performed. For example, in a case where an element composed of ZnO as a main component and Bi.sub.2 O.sub.3, and which contains many kinds of metallic oxides such as Sb.sub.2 O.sub.3, MnCO.sub.3, Cr.sub.2 O.sub.3, Co.sub.2 O.sub.3, SiO.sub.2, NiO, B.sub.2 O.sub.3, Al(NO.sub.3).sub.3 and so on, there is a problem, in some cases, that the .gamma.-type Bi.sub.2 O.sub.3 is hardly formed in the ZnO element and the coefficient .alpha. becomes small when the sintered ZnO element is cooled in the first heat-treatment at the cooling speed of 300.degree. C./h as described in the conventional technology.
For the above-noted reason, in the conventional technology, a multi- component ZnO element used in a high applying voltage environment is insufficient in reliability in withstanding discharge capacity and in voltage applying lifetime characteristics.
An object of the present invention is to provide a method of fabricating a high limiting voltage and stable ZnO element and an artester therewith where the ZnO element is high in reliability with respect to the withstanding discharge capacity characteristic and the voltage applying life time characteristic, and which does not deteriorate in its characteristics.
In order to attain the above objects, according to the present invention, there is provided a method of fabricating a voltage non-linear resistor which comprises, in a process for mixing a raw material containing ZnO as a main component with additives to produce voltage non-linearity such as Bi.sub.2 O.sub.3, Co.sub.2 O.sub.3, MnO, Sb.sub.2 O.sub.3, Cr.sub.2 O.sub.3, NiO, SiO.sub.2, GeO.sub.2, Al(NO.sub.3).sub.3, B.sub.2 O.sub.3 and so on, through a process for mixing the additives without SiO.sub.2 and GeO.sub.2 or a process for mixing the additives without at least one of SiO.sub.2 and GeO.sub.2, calcining the mixture in atmospheric environment at a temperature of 800.degree. to 1000.degree. C., milling the calcined mixture to obtain composite oxide, mixing and granulating the composite oxide with SiO.sub.2, 1% to 50% by weight (wt %) against the total weight of the composite oxide to form a compacted body. The method further comprises a process for sintering the compacted body at a temperature of 1150.degree. to 1300.degree. C., a process of a first heat-treatment which is composed of cooling the sintered body below 300.degree. C., after that heating it to 800.degree. to 950.degree. C. and maintaining that temperature for 1 to 3 hours, then cooling it below 300.degree. C., a process of a second heat-treatment which is composed of heating it again to 650.degree. to 900.degree. C. and keeping the temperature for 1 to 3 hours, then cooling it to room temperature, wherein the cooling speeds after keeping the sintered element in the first and second heat-treatment are below 100.degree. C. and 150.degree. C., respectively.
Another aspect of preferred embodiments of the present invention is to provide an apparatus for fabricating granular powder which comprises a mechanism for calcining additives such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, MnCO.sub.3, Cr.sub.2 O.sub.3, Co.sub.2 O.sub.3, Sio.sub.2, NiO, B.sub.2 O.sub.3 and so on and for weighing a milled composite oxide and SiO.sub.2, a mechanism for mixing the weighed composite oxide and SiO.sub.2, a mechanism for weighing ZnO and Al(NO.sub.3).sub.3, and a mechanism for mixing mixed powder of said composite oxide and said SiO.sub.2 and mixed powder of ZnO and Al(NO.sub.3).sub.3 to fabricate a granular powder.
Another aspect of preferred embodiments of the present invention is to provide an arrester constructed by placing the ZnO element, formed as a disk-shaped or cylinder-shaped sintered body and having an electrode at its end surface except its peripheral surface manufactured through the above-mentioned method, into an insulator tube or insulator tank.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.